The field of the invention pertains to fasteners and means to ascertain the holding or clamping force of an installed fastener. In particular, the invention pertains to threaded fasteners for retaining two or more parts together, however, the invention is not limited to threaded fasteners but may also apply to non-threaded fasteners where sufficient clamping force must also be assured.
In the typical installation the torque required to turn a threaded fastener is used to indirectly indicate the clamping force or assure that sufficient clamping force has been applied. Unfortunately, the torque measured is affected by a variety of parameters only partially controllable. Inaccurate, mismatched, or cross threaded engagement will result in a false torque indication, as will contaminants such as oils, greases or burrs and abrasive particles. Thus, a torque that appears to meet specifications may in actuality be insufficient or well beyond the torque required for the proper clamping force.
In order to avoid the difficulties with torque measurements, more direct means of determining the clamping force have been developed. In particular, modifications to the bolt or the washer under the bolt head have been developed and met limited success. U.S. Pat. No. 4,773,272 discloses a washer with a tab disposed in a radial groove. The deflection of the tab as the bolt is tightened provides a measure of the bolt clamping force.
Japanese Patent Document 54-71676 discloses a hollow bolt with a central pin in the bolt. Above the head of the bolt are a pair of measuring rings separated by a gap. As the bolt is tightened the gap closes thereby closing an electric circuit when the proper bolt tension is reached.
U.S. Pat. No. 4,294,122 and U.S. Pat. No. 3,969,960 disclose ultrasonic means to measure bolt tension. Both disclose contact with the head of the bolt, however, the former requires use of a transducer in the bolt head and the latter requires bolts in which the stretch constant is known. U.S. Pat. No. 4,823,606 discloses a bolt having a diaphragm transducer and strain gages disposed within the bolt to sense loading on the bolt.
A special washer that plastically deforms or crushes under the bolt head at the desired clamping force is disclosed in U.S. Pat. No. 4,333,220 with means to sense the plastic deformation and automatically halt the tightening means. U.S. Pat. No. 4,359,906 discloses a device for tightening a screw into bone and determining the applied torque to prevent overtightening of the screw and damage to the bone.
Most of the devices above require substantial modification of the bolt or washer which adds considerable expense and limits the applications for the bolts and washers. In addition a complicated measuring device is required in most of the examples. Thus, means are needed to directly measure bolt tension or clamping force in standard bolt and washer installations without significant additional expense.